These are my steps from flashing Debian Jessie to having a lan- and wifi router on the B3, including DHCP and firewall. I have tried countless times to follow a guide or an idea, but kept loosing the big picture and forgetting the steps to success and failure. So I started to write down the steps and finally ended up with this guide :D

B3 server setup with Debian 8Install image on B3
1. Put Debian image on usb-stick.
2. Unplug power from B3.
3. Insert Debian usb-stick.
4. Plugin power while pressing the reset switch (release the switch after led lights up green).
5. Wait while Debian 8 is being installed on B3.
6. When the led turns blue, the install has finished and the B3 is running Debian 8 :-)

Setup the laptop to ssh into B3
Find inspiration here: http://askubuntu.com/questions/691986/network-manager-shared-connection-and-manual-settings-of-the-interface
1. Shell command: sudo apt-get install dnsmasq-base
2. Remove dnsmasq because it conflicts with NetworkManager’s ‘Shared Connection’.
Shell command: sudo apt-get remove dnsmasq
3. Open Networkmanager → Edit Connections → Add → Connection Type 'Ethernet' → Create → Give your new connection a name. We'll use the name 'Shared Connection' for this example → IPv4 tab → Shared to other computers (This is basically a dead-simple NAT (Network Address Translation), or Internet connection sharing, built right in to NetworkManager) → save
4. Select the connection Shared Connection in Networkmanager and let B3 connect to your laptop.
5. When a connection has been established, note the IP-address of B3 by looking into /var/log/syslog (or similar log file). This can take some time before the IP-address of B3 shows up in log ~ 5-10mins.
6. Open terminal and run ssh excito@x.x.x.x (x.x.x.x = IP-address from step 5). Password is excito
7. Run as root (su). Password is excito

Enable forwarding
6. Edit /etc/sysctl.conf and uncomment the line net.ipv4.ip_forward=1
7. Reboot the B3 (issue the reboot command in shell or press reset switch until led change color to purple).

Connect again
8. Re-open the Networkmanager on your laptop → Edit Connections → add a new connection and name it ‘Troubleshooting’.
9. Select the tab IPv4 Settings and chose method: Manual
10. Add the following:Address = 192.168.10.2
Netmask = 255.255.255.0
Gateway = 192.168.10.1
11. In Networkmanager select the connection Troubleshooting and let it connect to B3.
12. Open terminal and ssh excito@192.168.10.1 with password excito
13. Run as root (su). Password is excito

Setting up dnsmasq
14. Shell command: apt-get install dnsmasq
15. Edit /etc/dnsmasq.conf and add this at the end of the file:interface=eth1
dhcp-range=eth1,192.168.10.100,192.168.10.200,12h#interface=wlan0
#dhcp-range=wlan0,192.168.11.100,192.168.11.200,12h
16. Edit /etc/resolv.conf and add this to the first line:nameserver 127.0.0.1 (maybe this step should be removed?, when I reboot the B3 this line is gone...)
17. Shell command: /etc/init.d/dnsmasq restart
18. Reboot the B3 (issue the reboot command in shell or press reset switch until led change color to purple).

However, I think my guide is a little rough to be in a wiki, especially the part in the beginning about setting up the host computer for sharing connection to ssh into the B3. Also the guide has not been tested thoroughly (only two times...).